>'<*?,  f 


cx 


ON  THE  DEVELOPMENT  OF  VISUAL  PERCEPTION 
AND  ATTENTION.1 

By  Harold  Griffing,  Ph.  D. 

(From  the  Psychological  Laboratory  of  Columbia  College.) 


Reprinted  from.  The  American  Journal  of  Psychology , Vol.  VII,  Ho.  2. 

It  is  well  known  that  a number  of  simultaneous  impres- 
sions on  the  retina  can  be  perceived  when  the  time  of  expos- 
ure is  so  short  as  to  exclude  successive  attention.2  The 
extensive  threshold,  as  the  greatest  number  of  objects  thus 
seen  may  be  called,  varies  with  different  individuals  and 
doubtless  many  other  conditions.  The  object  of  the  experi- 
ments now  to  be  described  was  to  determine  some  of  these 
conditions,  especially  those  relating  to  the  age  and  develop- 
ment of  the  observer.  As  the  accuracy  of  perception  clearly 
depends  upon  attention,  I hoped  also  to  obtain  some  data 
bearing  upon  the  development  of  voluntary  attention. 

The  apparatus  used  was  constructed  on  the  same  principle 
as  the  gravity  chronometer  of  Cattell  and  the  tachistoscope  of 
Yolkmann  and  Wundt  (see  Figure  1).  The  objects  to  be  per- 
ceived, which  were  letters,  were  pasted  on  white  cardboard. 
This  was  placed  on  a wooden  upright  board.  In  front  of  this 
board  is  a movable  screen  of  cardboard  AB,  with  a rectangu- 
lar opening,  which,  when  allowed  to  fall  past  the  objects, 
exposes  them  for  the  time  taken  by  the  opening  in  passing. 
The  screen  is  let  fall  by  the  operator,  who  pulls  the  string 
attached  to  the  clasp  M.  The  noise  of  falling  is  greatly  less- 
ened by  layers  of  felt  in  the  screen  holder  The  entire 
apparatus  is  hidden  by  a curtain  CD.  with  a rectangular 
opening  OL , where  the  stimulus  HK  appears.  The 


‘The  experiments  to  be  described  were  partially  planned  in  con- 
junction with  Mr.  S.  H.  Rowe,  formerly  Fellow  in  Education,  Co- 
lumbia College.  On  account  of  absence  in  Europe,  Mr.  Rowe  was 
unable  to  continue  the  research. 

2For  previous  investigations  on  the  subject,  see  Cattell,  Philosoph  - 
ische  Studien , III,  or  Brain,  XXXI. 


f z 2.  v ^ 


228 


GRIFFING : 


advantage  of  the  curtain  is  that  an  observer  is  ignorant  of  the 
movement  of  the  screen,  except  as  he  perceives  the  objects 
exposed  or  the  white  background  upon  which  they  appear. 
In  the  centre  of  the  opening  of  the  curtain,  and  attached  to  the 
curtain  by  thread,  is  a small  white  cross  F , serving  as  a fixa- 
tion point.  The  entire  apparatus  is  painted  black.  The 
time  of  exposure  in  these  experiments  was  second. 1 

With  this  instrument  the  writer  made  a large  number  of 
experiments  on  school  and  college  students,  mostly  from  tho 


M 


l\J 


\ 


Fig.  1. 

Horace  Mann  School  of  the  Teachers’  College  and  the  School 
of  Arts  in  Columbia  College.  Ten  successive  experiments 
were  made  on  each  group  of  observers,  the  number  in  a group 
varying  from  ten  to  thirty.  The  groups  were  arranged  in 
approximately  a triangular  shape,  so  that  all  could  see  dis- 
tinctly anything  near  the  fixation  point.  In  each  experiment 
the  stimuli  were  six  capital  letters  arranged  in  two  rows  of 
three  each,  and  presenting  the  appearance  of  an  approximate 
square.  The  letters  were  printed  for  the  purpose,  and  were 
48  mm.  in  height.  The  combinations  of  letters  were  as  fol- 


'This  time  was  obtained  theoretically  from  the  formulae  of  me- 
chanics. 


DEVELOPMENT  OF  VISUAL  PERCEPTION 


229 


lows : (i)  YOG  LNA,  (ii)  EYX  MHK,  (iii)  UJK  ZWD, 

(iv)  NXA  GFO,  (v)  DRK  LSI,  (vi)  YZB  CTP,  (vii) 
JNW  HYE,  (viii)  TSX  LFA,  (ix)  CDI  RGK,  (x)  OBP 
UJM.  They  were  selected  so  as  to  avoid,  if  possible, 
any  decided  difference  in  the  legibility  of  the  different  groups. 
For  this  purpose  I used  the  results  obtained  by  Prof.  Cattell 
for  the  legibility  of  letters.1  In  conducting  the  experiment 
the  observers  were  told  to  look  at  the  fixation  point  when 
the  signal  “ ready 79  was  given,  and  to  continue  to  look  at  the 
fixation  point  until  the  letters  appeared,  when  they  were  to 
write  down  what  letters  they  saw.  They  did  not  know  how 
many  letters  would  appear,  and  care  was  taken  that  they 
could  not  know  when  to  expect  the  stimulus.  The  purpose  of 
this  was  to  test  the  observer’s  powers  of  prolonged  attention. 
If  he  was  not  attending,  at  least  to  the  extent  of  looking  at  the 
apparatus,  he  could  not  see  anything. 

The  fact  that  some  observers  were  necessarily  in  more 
favorable  positions  for  seeing  than  others  may,  we  think,  be 
neglected.  In  the  experiments  on  three  different  groups 
those  unfavorably  seated  had  about  as  good  records  on  the 
average  as  those  favorably  seated. 

In  the  experiments  on  the  college  students,  the  first  that 
were  made,  the  intervals  of  time  between  the  signal  and  the  ap- 
pearance of  the  stimuli  were  1,  £,  1J  and  £ minutes,  and  the 
same  for  the  second  five  of  the  ten  experiments.  As  the 
results  showed  no  effect  of  fatigue,  and  as  the  writer  wished 
to  obtain  some  data  on  the  subject,  in  the  succeeding  experi- 
ments on  pupils  of  the  H.  M.  S.2  the  intervals  were  increased, 
being  J,  3,  1,  4 and  2 minutes. 

I now  give  the  results  of  the  experiments  for  the  different 
classes,  I being  the  lowest  primary  of  the  H.  M.  S.  The 
high  school  pupils  include  four  groups  from  the  H.  M.  S. 
and  two  private  schools,3 4  and  the  college  students  include 
five  groups  from  Barnard  and  Columbia  Colleges.  4 


’Cattell,  op.  tit. 

’I  will,  for  convenience,  use  this  abbreviated  form  for  the  Horace 
Mann  School. 

3Mr.  Browning’s  school  for  boys  and  Miss  Gibbons’  school  for 
girls,  both  in  New  York. 

4I  take  pleasure  in  here  expressing  my  thanks  to  the  school  and 
college  officers  who  have  given  me  the  opportunity  to  make  these 
tests. 


230 


GRIFFIN  G : 


Table  I. 

Average  Total  Numbers  of  Letters  Seen  by  Classes. 


N. 

C. 

s 

MV. 

R. 

MV. 

MAX. 

MIN. 

a 

8 

22 

I 

8 

6 

3 

3 

17 

0 

.4 

16 

II-III 

13 

8 

6 

4 

19 

0 

.4 

12 

IV 

16 

6 

7 

3 

13 

1 

.4 

17 

V 

18 

5 

14 

4 

22 

3 

.8 

17 

VI 

22 

10 

12 

5 

25 

0 

.5 

23 

VII 

19 

7 

14 

5 

23 

0 

.7 

23 

VIII 

25 

7 

21 

6 

37 

8 

.8 

84 

High 

30 

5 

23 

6 

47 

4 

.8 

75 

Coll. 

32 

6 

29 

6 

59 

11 

.9 

N.  = number  in  class. 

C.  = class. 

S.  = average  of  total  number  of  letters  written  down  as  seen  in 
ten  trials,  six  letters  being  given  in  each  trial. 

R.  = average  of  total  number  seen  correctly. 

MV.  = mean  variation  of  S.’s  or  R.’s  of  the  individual  averages 
from  the  averages  of  the  groups. 

MAX.  = maximum  of  total  number  seen  correctly  by  any  in- 
dividual. 

MIN.  = minimum  of  total  number  seen  correctly  by  any  individual. 

In  table  II  are  given  the  results  for  students  classified 
according  to  age.  The  probable  errors  of  the  values  of  R.  are 
given  in  the  R.  column,  preceded  by  the  sign  ±.  The  MY.’s 
for  the  R.’s  only  are  given. 

Table  II. 


Average  Total  Numbers  of  Letters  Seen  for  Different  Ages. 


N. 

AGE. 

S. 

R. 

MV. 

MAX. 

MIN. 

R 

r 

39 

7-9 

11 

4 ± .4 

3 

33 

0 

.4 

77 

10  - 12 

20 

13  ± .3 

3 

32 

0 

.6 

73 

13  - 15 

24 

18  ± .6 

6 

37 

0 

.7 

132 

161  + 

32 

27  ± .4 

6 

59 

8 

.8 

‘Most  of  these  observers  were  from  16  to  18  years  of  age. 


DEVELOPMENT  OF  VISUAL  PERCEPTION. 


231 


From  the  above  tables  it  is  evident  that  the  extensive  thresh- 
old, or  ability  to  receive  and  retain 1 a number  of  simultaneous, 
retinal  impressions,  is  a function  of  individual  growth,  reach- 
ing its  maximum  only  when  the  observer  is  fully  developed. 2 
The  average  number  seen  correctly  in  one  trial  by  the  adult 
observers  was  about  three,  whereas  children  from  7 to  9 saw 
but  one.  These  numbers  would  be  less  if  the  element  of  chance 
were  eliminated. 3 They  would,  on  the  other  hand,  probably  be 
somewhat  greater  under  more  favorable  conditions.  In  my 
experiments  the  observers  did  not  know  just  when  to  expect 
the  exposure,  and,  moreover,  we  must  not  expect  such  experi- 
ments to  give  results  as  exact  as  those  of  the  laboratory. 
Practice  increases  the  extensive  threshold,  and,  as  is  shown 
by  the  columns  MY.  MAX.  and  MIX.,  great  individual  varia- 
tions were  observed.  Yery  few  adult  observers  saw  five  letters 
on  the  average,  and  some  adults  saw  but  two.  The  tendency 
to  guess  seems  to  decrease  with  maturity.  The  average  mean 
variations  of  the  separate  observations  of  the  individual  ob- 
servers from  their  averages  were  found  to  be  about  the  same 
for  the  younger  as  for  the  older  students.  But  the  relative 
variation  (^)  for  the  children  of  10  to  12  was  found  to  be 
about  double  that  of  the  students  of  16  and  over.  This  is 
what  we  should  expect,  as  it  is  a matter  of  common  observa- 
tion that  children  are  deficient  in  power  of  constant  attention. 

A question  of  considerable  interest  is  the  relation  of  the 
pupil’s  range  of  perception  to  his  intellectual  capacity  as 
judged  by  his  teacher.  I found  that  those  rated  A for  mental 
capacity  by  the  teachers,  on  an  A,  B,  C basis,  had  some- 
what higher  averages  than  the  others,  and  out  of  the  twelve 
best  observers  (four  from  each  age  group)  eight  were  rated 
A and  but  one  C.  There  are,  however,  marked  exceptions. 
One  young  lady  of  18,  known  by  the  writer  to  be  a brilliant 
student,  saw  but  sixteen  letters  out  of  the  sixty,  and  in  no 
case  more  than  three.  Those  marked  A by  their  teachers 
for  attention  in  class  also  excelled  the  others,  but  here  also  I 
found  decided  exceptions.  Many  pupils  must  have,  therefore, 
good  powers  of  attention  even  when  they  show  no  evidence 
of  them  to  their  teachers.  Xo  difference  was  found  between 


‘I  will  use  this  expression  hereafter  in  the  sense  in  which  it  is 
here  used,  without  making  any  assumption  as  to  its  psychological 
interpretation. 

2The  same  result  for  auditory  memory  and  attention  was  found 
by  Bolton.  American  Journal  of  Psychology,  Vol  IV,  Xo.  3. 

3The  probability  of  correctly  guessing  any  one  letter  was  for  the 
older  students  about  one  third  of  the  probability  of  writing  down  a 
letter  correctly  as  found  in  the  experiments,  and  for  the  younger 
students  somewhat  greater. 


232 


GRIFFING  : 


the  girls  and  boys,  the  averages  closely  corresponding.  Other 
investigators  have  found  that  the  girls  tend  slightly  to  excel 
the  boys  in  the  tests  given.1  More  extended  experiments 
might,  however,  show  a difference. 

In  order  to  investigate  the  question  of  fatigue,  the  averages 
were  taken  for  each  of  the  ten  experiments  for  each  group. 
The  results  for  the  experiments  in  which  long  times  of  wait- 
ing were  used,  one  to  four  minutes,  were  somewhat  better 
than  those  in  which  short  times  were  used.  This  was  found 
for  both  series  of  times,  J,  3,  1,  4 and  2 minutes,  and  y1^,  1, 
J,  14  and  J minutes. 

But  fatigue  of  the  visual  centres  must  have  been  present, 
since  several  observers  complained  of  pain  in  the  eyes  and 
even  headache  from  the  strain  of  attention.  As  the  results 
were  not  appreciably  affected,  we  conclude  that  the  attention 
of  children  may  be  taxed  to  the  extent  of  causing  abnormal 
fatigue  without  any  marked  effect  on  the  accuracy  of  percep- 
tion. As  it  is,  moreover,  improbable  that  the  long  periods 
of  waiting  are  more  favorable  for  perception,  or  at  least  so 
much  so  as  is  indicated,  we  must  infer  that,  despite  precau- 
tions to  avoid  this,  there  are  decided  differences  in  the 
legibility  of  the  different  groups  of  letters.  Practice  cannot 
account  for  the  results  for  experiments  Y and  YI,  the  ac- 
curacy of  observation  for  Y being  about  30%  and  40%  greater 
than  for  YI  for  the  two  time  series.  Nor  can  we  ascribe  the 
greater  legibility  of  Y to  the  difference  in  fatigue,  for,  on  the 
one  hand,  in  the  case  of  the  H.  M.  S.  observers,  the  time  of 
waiting  for  V was  much  longer  than  for  YI ; on  the  other,  in 
the  case  of  the  college  students  and  some  others,  the  times 
for  both  were  very  short.2 *  We  infer,  then,  that  one  deter- 
mining factor  is  the  arrangement  of  the  letters.  But  in  neither 
case  was  a syllabic  combination  used,  and  the  combinations 
of  threes  certainly  do  not  make  u sense.’ 9 On  the  other 
hand,  YZB  and  CTP,  the  letters  used  for  YI,  have  much 
less  similarity  to  combinations  of  letters  than  in  actual  use 
than  have  DRK  and  LSI,  those  used  in  Y.  If  this  be  the  ex- 
planation, the  combinations  are  perceived  as  units  rather  than 
as  separate  and  distinct  objects  ; or  at  least  the  mind  tends 
to  perceive  them  so,  and  is  successful  in  proportion  to  the 
ease  with  which  the  perceptive  processes  of  the  separate 
letters  are  mutually  helpful.  From  this  it  would  follow  that 
we  do  not,  as  has  been  supposed,  see  several  things  at  once, 
but  see  the  given  stimuli  as  a unit  and  then  analyze  this  unit 


‘Bolton,  op.  cit.  J astro w,  Educational  Review , Dec.,  1891. 

2The  same  result  was  found  for  three  private  school  groups  not 

here  included,  as  the  times  were  not  the  same. 


DEVELOPMENT  OF  VISUAL  PERCEPTION. 


233 


into  its  components.  It  is  indeed  possible  that  since  the 
legibility  of  the  letters  depends  upon  the  distinctness  of  the 
retinal  images,  some  combinations  of  letters  may  not  be  so 
favorable  as  others  for  the  formation  of  distinct  retinal 
images  of  the  different  letters.  It  is  interesting  to  note  that 
in  other  experiments  in  which  fatigue  could  play  but  a small 
part,  the  relation  of  V and  VI  was  the  same  as  in  these. 1 

In  the  experiments  described  above,  although  verbal  and 
syllabic  combinations  were  avoided,  several  observers  per- 
ceived certain  combinations  as  words.  Thus  TSX  LFA  was 
read  TEXAS,  three  times  ; OBP  UJM  was  read  JUMP,  once ; 
YOG  UNA  was  read  LONG,  once,  LONG  WAY,  once,  and 
YOU,  once  ; and  UJR  ZWD  was  read  ARE,  once.  In  a very 
few  instances  more  letters  were  written  down  than  were 
actually  given.  Three  observers  perceived  the  group  first 
given  as  ABC,  etc.,  the  first  letters  of  the  alphabet. 

A limited  number  of  experiments  were  made  with  the  same 
time  of  exposure,  but  with  this  difference,  that  one  letter  was 
exposed  instead  of  six.  The  letters  were  CRYSNIXOJA. 
The  times  of  waiting  were  J,  J,  and  £ minutes  for  the 

first  five,  and  the  same  for  the  second  five.  The  test  was  made 
on  twenty- three  high  school  pupils  and  on  sixteen  pupils 
of  grade  II  in  the  primary  department  of  the  H.  M.  S.  In  the 
230  observations  (10x23)  of  the  high  school  pupils,  only  seven 
letters  were  written  down  incorrectly  ; and  in  the  160  obser- 
vations of  the  primary  class  only  twenty- eight  were  not  rightly 
perceived.  The  results  show  that  the  inability  of  the  younger 
children  to  perceive  the  letters  when  six  are  given  is  due,  not 
merely  to  the  brief  period  of  exposure,  but  also  to  the  com- 
plexity of  the  stimulus. 

A few  experiments  were  also  made  in  which  six  colors  were 
used  instead  of  letters.  As  the  results  were  approximately 
the  same,  we  may  conclude  that  the  inferences  based  upon 
the  tests  with  letters  are  valid  for  the  perception  of  objects  in 
general  and  not  for  letters  only. 

The  experiments  I have  described  were  all  made  with  ^ 
second  exposure.  I will  now  describe  other  experiments  in 
which  the  time  of  exposure  was  one  second.  These  experiments 
were  made  only  on  classes  I- VIII  inclusive  of  the  H.  M.  S.  and 
a group  of  high  school  students.  The  same  apparatus  was 
used,  but  was  of  course  adapted  to  the  change  of  time.  This 
was  done  by  attaching  a heavy  weight  W to  a cord  which  ran 
over  a friction  pulley  P and  was  attached  to  the  drop  screen 
AB  (see  Figure  1).  When  the  screen  was  allowed  to  fall,  its 
velocity,  and  consequently  the  time  of  exposure,  was  regulated 


1 See  later  experiments  with  one  second  exposure. 


234 


GRIFFIN G : 


by  the  weight  on  the  principle  of  Atwood’s  machine.  The 
same  combinations  of  letters  were  used  as  in  the  TO  second 
experiments,  but  several  months  elapsed  between  the  two 
series  of  experiments.  The  experiment  was  conducted  in  the 
same  manner,  but  short  times  of  waiting  only  were  used,  5, 
30,  15,  45  and  10  seconds,  and  the  same  repeated  for  the 
second  5 experiments. 

In  the  following  table,  I give  the  results  of  these  experiments. 
The  letters  mean  the  same  as  in  table  I. 


Table  III. 

Average  total  numbers  seen  with  1 second  exposure  for  different 
classes. 


N. 

C. 

s. 

MV. 

R. 

MV. 

MAX. 

MIN. 

R 

T~ 

19 

I 

20 

7 

17 

5 

36 

7 

.85 

17 

II 

30 

5 

26 

4 

38 

20 

.87 

10 

III 

37 

5 

33 

5 

49 

20 

.89 

20 

IV 

35 

5 

28 

6 

48 

14 

.77 

15 

! V 

40 

7 

36 

6 

43 

25 

.90 

22 

| VI 

44 

6 

38 

5 

54 

28 

.82 

13 

VII 

51 

8 

44 

8 

58 

29 

.86 

11 

! VIII 

50 

1 8 

47 

7 

59 

31 

.94 

10 

High 

59 

2 

55 

6 

60 

40 

.93 

The  results  show  that  the  range  of  perception  for  one  second 
exposure  also  depends  upon  individual  growth.  The  brighter 
students  tend  to  excel  in  these  as  in  the  ^ second  tests.  This 
was  found  by  two  distinct  methods,  as  before.  There  does  not 
seem,  however,  to  be  any  close  connection  between  the  two 
tests.  The  best  observers  in  the  one  second  tests  include 
many  that  had  poor  records  in  the  previous  tests.  But 
temporary  conditions  would  cause  some  variation  in  the  same 
observer. 

As  in  the  TV  second  experiments,  variations  were  found  in 
the  accuracy  of  perception  for  the  different  combinations  of 
letters.  As  the  times  of  waiting  were  very  short  and  as  close 
attention  was  not  necessary  in  order  not  to  miss  the  letters, 
these  differences  cannot  be  ascribed  to  fatigue. 

The  relative  variation  was  found  to  be  fairly  constant  and  to 
be  but  little  greater  for  the  children  of  the  primary  classes  in  the 


DEVELOPMENT  OF  VISUAL  PERCEPTION. 


235 


♦ one  second  tests  than  for  the  high  school  and  college  students 
in  the  second  tests.  But  the  relative  variation  for  the 
younger  pupils  is  at  the  same  time  much  less  in  the  one 
second  than  in  the  second  tests.  This  is  what  one  might 
expect,  since,  when  the  time  of  exposure  is  as  long  as  one 
second,  continuous  concentration  of  the  attention  is  not  neces- 
sary as  in  experiments  in  which  short  times  of  exposure  are 
given. 

I have  up  to  this  point  endeavored  to  make  no  assumptions  as 
to  the  interpretation  of  the  experiments  described.  The  sim- 
plest interpretation  is  that  the  extensive  threshold  measures 
the  number  of  objects  that  can  be  simultaneously  grasped  by 
consciousness.  It  is  probable,  however,  that  the  process  is 
quite  complex.  In  fact  the  results  found  for  the  different  com- 
binations do  not  favor  so  simple  an  interpretation.  The  accur- 
acy of  the  result  may  depend  upon  the  reproductive  processes 
involved,  and  the  analysis  of  the  memory  image.  It  is  possible 
also  that  the  sensitiveness  of  the  retina  is  a determining  factor. 

Whatever  be  the  exact  nature  of  the  mental  process,  it  is 
certain  that  the  accuracy  of  perception  and  reproduction  will 
depend  to  some  extent  upon  the  attention.  In  fact  Wundt 
identifies  the  extensive  threshold  with  attention.1  From 
this  point  of  view  the  results  of  the  y1^  second  experiments 
would  measure  the  capacity  of  the  observers  for  concentrated 
attention.  It  is  quite  true  that  attention  is  necessary  in  order 
to  see  the  letters,  but  the  assumption  that  it  is  the  only  factor 
is  unwarranted.  The  results  of  the  experiments  certainly  are 
not  favorable  to  such  an  interpretation.  No  decrease  in  the 
number  of  letters  seen  was  found  for  the  longest  times  of 
waiting,  which  were  such  as  to  cause  decided  fatigue.  Then 
many  bright  students  proved  to  be  poor  observers,  and  it  is 
improbable  that  students  that  excelled  in  their  studies  would 
be  deficient  in  their  powers  of  attention.  But  even  if  we  assume 
that  the  experiments  measure  the  attention,  they  do  not  ne- 
cessarily measure  the  capacity  for  attention.  Those  interested 
in  the  experiments  and  desiring  to  excel  would  attend  more 
closely  than  others.  It  is  possible  that  the  general  superiority 
of  the  brighter  students  may  be  due  to  these  causes;  for 
children  of  the  most  active  minds  would  be  most  interested 
in  novel  experiences.  But  although  we  cannot  assume  that 
the  average  number  of  letters  seen  by  an  observer  measures 
his  powers  of  attention,  the  mean  variation  from  the  average 
of  the  numbers  seen  in  the  different  experiments  is  presumably 
due,  principally  at  least,  to  variations  in  the  attention. 


1 Wundt’s  expression  is  “ the  extent  of  apperception,”  Orundziig 
d.  Phys.  Psy.j  IV*"  Aufl.,  II,  287. 


236 


GRIFFING. 


With  regard  to  the  one  second  experiments,  the  conditions 
are  more  complex  than  in  the  others,  but  at  the  same  time 
conform  more  closely  to  those  of  the  preceptions  of  ordinary 
experience.  The  number  of  letters  seen  doubtless  depends 
upon  the  degree  of  attention,  but  it  also  depends  upon 
the  readiness  with  which  the  attention  is  fixed,  and  the 
time  of  perception.  The  development  of  the  visual  memory 
may  affect  the  results  in  both  series  of  experiments.  That 
there  is  a radical  difference  between  the  processes  involved  is 
made  probable  by  the  fact  that  some  that  excelled  in  one  test 
did  poorly  in  the  other. 


